1. Field of the Invention
This invention relates to a dry etching method employed for the preparation of semiconductor devices. Note particularly, it relates to a dry etching method for dry etching a layer of a silicon compound at a high etchrate with excellent resist selectivity and silicon underlying layer selectivity with the least risk of pollution by particles.
2. Description of the Related Art
In keeping up with the high integration and high performance of semiconductor ,devices, such as VLSIs and ULSIs of recent origin, technical demands placed on the methods for dry etching of a layer of a silicon compound, exemplified by silicon oxide, are becoming increasingly strict.
That is, the wafer size and the device chip area are increased with increase in the degree of integration, whilst the degree of refinement of the pattern to be formed is also increasing. Consequently, for assuring etching uniformity in a wafer surface, the tendency is towards using a wafer-by-wafer type etching system in preference to the conventional batch type etching system. For maintaining productivity comparable to that achieved heretofore, it is mandatory to elevate the etch rate significantly.
Under the current status of the art in which, for increasing the operation speed and reducing the size of semiconductor devices, the depth of junction of the impurity diffusion region is becoming shallow and a variety of deposited films are reduced in thicknesses, a demand is presented for an etching technique which is more excellent in underlying layer selectivity and less subject to damages than the conventional etching technique. This is the case with etching a silicon oxide interlayer insulating film which is performed with a silicon substrate or a polysilicon layer as an underlying layer for forming a contact to an impurity diffusion region formed in a semiconductor substrate or to a source-drain region of a pMOS transistor used as a resistor load for SRAMs.
It is also mandatory to improve resist selectivity. In submicron devices, even the slightest dimensional loss due to recession of the resist material is not tolerated.
However, such properties as high etchrate, high selectivity and low damages are matters of trade-off such that it is extremely difficult to arrive at an etching process which will satisfy these requirements simultaneously.
Heretofore, when etching a layer of a silicon compound, such as silicon oxide, while maintaining high selectivity against a layer of the silicon-based material, CHF.sub.3, a CF.sub.4 /H.sub.2 mixed system, a CF.sub.4 /O.sub.2 mixed system or a C.sub.2 F.sub.6 /CHF.sub.3 mixed system has been customarily employed as an etching gas. These etching gases are unanimously based on fluorocarbon based gases having a C/F ratio (the ratio of the number of carbon atoms to that of fluorine atoms) of not less than 0.25. These gas systems are used because (a) the carbon contained in the fluorocarbon gases generates C--O bond having a high interatomic bond energy on the surface of the silicon oxide layer to disrupt or otherwise weaken the Si--O bond of the silicon oxide layer, (b) CF.sub.x.sup.+ ions, above all, CF.sub.3.sup.+ ions, which are main etchants for the silicon oxide layer, may be generated, and (c) a relatively carbon-rich atmosphere is produced in a plasma and hence oxygen in silicon oxide is removed in the forms of CO or CO.sub.2, whereas a carbonaceous polymer is deposited on the surface of the layer of the silicon-based material, due to contribution of C, H or F contained in the gas, thereby lowering the etch rate and improving selectivity against the silicon-based layer.
Meanwhile, the above mentioned addition gases H.sub.2 or O.sub.2 are used for controlling selectivity and is capable of reducing or increasing the amount of generation of F.sup.* ions, respectively. That is, these addition gases are effective in controlling the apparent C/F ratio of the etching system.
On the other hand, the present Assignee has proposed in JP Patent Publication KOKAI NO. 3-276626 a method for dry etching of a layer of a silicon compound using a saturated or unsaturated straight-chain higher fluorocarbon gas having two or more carbon atoms. With this prior-art dry etching method, higher fluorocarbon gases, such as C.sub.2 F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.10 or C.sub.4 F.sub.8, are used for efficient generation of CF.sub.x.sup.+ for raising the etchrate However if the higher fluorocarbon gas is used alone, resist selectivity or silicon underlying layer selectivity can not be increased. Although a high etchrate is achieved when a silicon oxide layer on a silicon substrate is etched using, for example, C.sub.3 F.sub.8 as an etching gas, resist selectivity is as low as about 1.3 and resistance to etching falls short while the dimensional loss is produced due to pattern edge recession. Resistance to overetching also falls short because silicon selectivity is of the order of 4.1. For overcoming these drawbacks, two-stage etching is used in the above described prior art method in such a manner that etching with the straight-chain higher fluorocarbon gas alone is continued until immediately before the underlying layer is exposed, and etching of the remaining portion of the layer of the silicon compound is performed using the above gas added to by a hydrocarbon based gas for promoting deposition of the carbon-based polymer.
However, under the current status of the art in which the design rule of the semiconductor devices has been refined to a higher extent the dimensional loss in connection with the etching mask is hardly tolerated, such that even with the two-stage etching as described above, it becomes necessary to improve selectivity further during the first-stage etching. Since the effects of pollution by particles of the carbonaceous polymer are feared to become more acute in future with progress in the refinement of the semiconductor design rule, it is highly desirable that the amount of consumption of deposition gases such as hydrocarbon gases during the second stage etching be reduced to as small a value as possible.
In view thereof, the present inventors have already proposed a method of etching a layer of a silicon compound using a straight-chain fluorocarbon gas having at least one unsaturated bond per molecule while the wafer temperature is controlled to a temperature not higher than 50.degree. C. Since the straight-chain unsaturated fluorocarbon gas theoretically yields two or more CF.sub.x.sup.+ ions per molecule by discharge dissociation, silicon oxide may be etched at a high etchrate with the use of this etching gas. Polymerization of the carbonaceous polymer is accelerated because the etching gas contains unsaturated bonds in the molecule thereof and tends to produce highly active radicals on dissociation. Besides, deposition of the carbonaceous polymer is accelerated because the wafer temperature is controlled to 50.degree. C. or lower. The result is improved resist selectivity and silicon underlying layer selectivity.
In the above described method by the present inventors, there has also been disclosed a technique in which etching with the straight-chain unsaturated fluorocarbon gas alone is continued until the time the layer of the silicon compound is etched halfway and the remaining portion of etching is performed using the above mentioned straight-chain unsaturated fluorocarbon gas, added to by the hydrocarbon gas, as an etching gas. With this technique, the deposition gas is used simultaneously during the latter half of the etching for further improving selectivity against the underlying silicon layer.
There is also disclosed in JP Patent KOKAI Publication No. 16-0938 a technique of etching silicon oxide using a mixed gas of hexafluorobenzene (C.sub.6 F.sub.6) and tetrafluoromethane (CF.sub.4) as an etching gas. With this technique, which is based on the concept similar to that shown in the above described present Inventors' proposal it contemplated to generate is CF.sub.x.sup.+ in a plasma efficiently by using cyclic unsaturated higher fluorocarbon gases, as well as to promote polymerization of the carbonaceous polymer.
It is noted that as will be apparent from above, it has been necessary with the previously proposed technique of employing straight-chain unsaturated fluorocarbon gases or cyclic unsaturated fluorocarbon gases to use other addition gases simultaneously for achieving high selectivity.
On the other hand, with the technique of employing C.sub.6 F.sub.6, the etching gas can not be constituted by C.sub.6 F.sub.6 alone, as stated in the specification of the JP Patent Publication which has disclosed the technique. It is because a large quantity of CF.sub.x.sup.+ ions are produced in the plasma with the use of C.sub.6 F.sub.6 alone to promote polymerization of the carbonaceous polymer excessively to interfere with the progress of the etching reaction. For this reason, CF.sub.4 having the lowest C/F ratio among the fluorocarbon gases is mixed with C.sub.6 F.sub.6 for suppressing generation of CF.sub.x.sup.+.
Consequently, when the cyclic high fluorocarbon gases are employed, it is more meritorious to use such gas as may be used alone for improving etching stability and controllability.